1. Field of the Invention
The present invention is related to centrifugal blood pumps.
2. Description of the Prior Art
Centrifugal pumps have been use for many years to pump a wide variety of different fluid materials. In general, a centrifugal pump includes a pumping chamber with an inlet aligned with a rotational axis of the pump, an outlet adjacent the periphery of the pumping chamber, and an impeller mounted within the pumping chamber for rotation about the axis. The impeller in such pumps can be mounted on a drive shaft which extends outside the pumping chamber to a rotational drive source or the shaft can be mounted within the pumping chamber as a spindle about which the impeller rotates (rotatably driven by means other than the rotation of the shaft, such as a magnetic drive arrangement). In any case, as the impeller is rotated, it imparts centrifugal force and velocity to the fluid, thus pumping the fluid from the pump inlet to the pump outlet.
In recent years, centrifugal pumps have been used extensively for pumping blood during open heart surgery. The pumping of blood requires great care to avoid any damage to the red corpuscles, or any of the other constituents of the blood. Any practical blood pump useful as part of heart/lung bypass equipment during open heart surgery must deliver the requisite flow volumes under pressure, without damaging the blood being pumped.
In a centrifugal pump, and in particular in a centrifugal pump for pumping liquids such as blood, a fluid tight seal between the rotating part (e.g. the drive shaft) and the non-rotating part (e.g. the housing) is an important factor in the performance of the pump. It is critical that the seepage of any external fluid (gas or liquid) into the blood pumping chamber be closely controlled and minimized, specifically limiting exposure to outside air which could enter the pumping chamber.
In blood pumps which are driven by a magnetic drive arrangement, the shaft seal can be more easily isolated from exterior contamination. For example, in Dorman et al. U.S. Pat. No. 3,608,088 a magnetic rotor is connected to an impeller and is sealed within the same housing as the impeller with the blood acting as a lubricant. In Rafferty et al. U.S. Pat. No. 3,647,324, the impeller is not shaft driven. The impeller is provided with magnetic components which coact with windings positioned concentrically about the impeller in the pump housing. In Reich et al. U.S. Pat. No. 4,135,253 the impeller is shafted to a magnetic rotor. Saline solution is directed past the rotor to lubricate it, with the saline solution being maintained at a higher pressure than the blood so that if there is leakage through a seal adjacent the impeller (between the pumping chamber and the magnetic rotor chamber), the saline solution will seep into the blood rather than vice versa.
Clausen et al. U.S. Pat. No. 4,606,698 and Clausen et al. U.S. Pat. No. 4,589,822 show a centrifugal pump having an impeller which is shaft driven via magnetic coupling. The pumping chamber is isolated from the magnetic rotor secured to the impeller shaft and the impeller shaft bearings by a shaft seal, and the chamber housing the magnetic rotor is otherwise sealed to the atmosphere to prevent the migration of contaminants (e.g., air bacteria, etc.) into the pumped blood from the outside of the pump. If a seal failure occurred in the device of these Clausen et al. patents, the quantity of air which could enter the pumping chamber is limited to that amount contained in the magnetic rotor chamber.
All of the devices of the above-mentioned patents have magnetic components imbedded in a rotating part which is attached to the pump impeller. Rotating motion is coupled magnetically to the pump impeller by placing the impeller in close proximity to a corresponding set of magnets which are attached to the rotating part of a suitable drive system or, in the case of the Rafferty et al. U.S. Pat. No. 3,647,324, by energizing the windings about the pump impeller. Recently there has been an increased emphasis on cost reduction in medical equipment and, as a result, a move to low-cost, single-use disposable equipment whenever possible. Bearings and magnetic components are relatively expensive, and thus lead to high costs for the disposable portion of a centrifugal blood pump which is magnetically driven. The elimination of the magnetic assembly and bearings from the disposable portion of such a pump would significantly reduce the cost associated with the pump.
Essentially, two problems have stood in the way of driving a centrifugal blood pump impeller by direct connection to the drive shaft of a motor: (1) the inability to provide a shaft seal which would retain pressure integrity for the period of time required for pump use, and (2) the danger of potentially fatal consequences, because of air inspiration, should that seal fail.
It has been proposed that a centrifugal pump for medical use have a shaft driven impeller wherein the shaft is mechanically coupled to a drive source, as opposed to a magnetic coupling. Rafferty et al. U.S. Reissue Pat. No. Re. 28,742; Kletschka et al. U.S. Pat. No. 3,864,055; Rafferty et al. U.S. Pat. No. 3,957,389; Rafferty et al. U.S. Pat. No. 3,970,408 and Rafferty et al. U.S. Pat. No. 4,037,984 show examples of these types of pumps. None of these patents, however, discloses a satisfactory arrangement for effectively sealing the pumping chamber from external contaminants adjacent the rotating impeller shaft, or at least limiting the scope of such possible contamination if a shaft seal should fail.